Electric recording and reproducing system



8, 1942. P. T. FARNSWORTH ELECTRIC RECORDING AND REPRODUCING SYSTEMFiled Feb. 15, 1940 FIG.]

I llllll/I/lll/I FIG.2

INVENTOR Patented Dec. 8, 1942 2,304,633 ELECTRIC RECORDING ANDREPRODUCING SYSTEM Philo T. Farnsworth, Fort Wayne, Ind., assignor toFarnsworth Television and Radio Corporation, a corporation of DelawareApplication February 15, 1940, Serial No. 319,111

13 Claims.

This invention relates to systems for recording and reproducing signals.

Electric recording and reproducing apparatus such as is conventionallyemployed embodies various undesirable features. One of the inherentfeatures of such apparatus is the reproduction of extraneous orbackground noises which substantially anatrromahe"quamyand fidelity ofthe reproduced recordings. Moreover, such apparatus has been subject tovarious other limitations effecting not only the quality and fidelity ofthe reproduction but the length of programs which may be recorded on arecord of given size.

In conventional phonograph disc recording and reproducing apparatus, forexample, it is customary to produce recordings of sound upon a waxrecord by inscribing a continuous trace thereon. This trace is producedby the motion of the wax record in conjunction with the movement of arecording stylus or cutter in accordance with electrical signals intowhich the sound is converted. In the reproduction of the recorded sound,a reproducing stylus or needle is caused to follow the trace of therecording stylus and the motion of the reproducing stylus is utilized toproduce electrical signals which are then converted into sound. In sucha system, the recording as well as the reproducing styli are at restwhen no impressions or signals are 7 being recorded or reproduced. Onlythe presence of a signal produces motion of the stylus.

The grooves inscribed by a recording stylus upon a wax record, whichserves as a master for the manufacture of other identical records, arenot as smooth as would be desirable, and a certain background noise,which is commonly referred to as needle scratching is usually present,when reproducing the recorded impressions.

In recording signals upon a physical medium, such as a wax disc record,the recording stylus is usually deflected to either side of apredetermined trace in accordance with the amplitude and frequency ofthe signal to be recorded. This method of recording is commonly referredto as lateral recording whereby the width of the grooves inscribed bythe recording stylus varies in accordance with the amplitude of thesignal being recorded. The recording stylus is guided along a spiralpath on a plane-wax record. The length of the recording is determined bythe number of spirals which can be placed upon a record of a givendiameter. The number of grooves, however, is determined by the Width ofthe individual groove which in turn is determined by the maximumdeflection of the recording stylus due to the greatest intensity of thesignal to be recorded. Hence, it is evident that for weaker signals alarge amount of the recording area is not utilized.

The object of the present invention is to provide a new and improvedmethod of and system for recording and reproducing impressions upon aphysical medium, which system will be free from the limitations andundesirable character istics of the systems of the prior art abovereferred to.

In accordance with the present invention, there is provided a system forrecording and reproducing impressions of audible and visible nature bythe agency of electrical signals corresponding thereto. The systemincludes a means for developing a carrier frequency signal whosefrequency is varied in accordance with a modulation signal representingthe impressions to be recorded. Means are provided to vary thecharacteristics of a physical medium in accordance with the modulatedcarrier frequency signal to produce a record of these impressions. Therecorded impressions are reproduced by a system comprising a meansresponsive to the varying characteristics which reproduces thefrequency-modulated carrier signal. A detector is provided for detectingthe carrier signal and the detected signal is applied to suitable meansfor reproducing the recorded impressions.

For a better understanding of the invention, together with other andfurther objects thereof, reference may be had to the followingdescription taken in connection with the accompanying drawing and itsscope will be pointed out in the appended claims,

In the accompanying drawing, Fig. 1 is a schematic diagram of arecording system, while Fig. 2 is a similar diagram of a reproducingsystem, both systems embodying the present invention and togetherrepresenting a system of recording and reproducing impressions ofaudible nature.

Referring now more particularly to Fig. 1 of the drawing, there is hereshown schematically a system for recording audible impressionscomprising a microphone l of conventional construction adapted toconvert audible impressions into electrical signals which are applied tothe control grid of an electron discharge tube 2.

For the purpose of generating a carrier frequency signal, there isprovided a generator of preferably asymmetrical electrical oscillationscomprising an oscillator tube 3 having a grid circuit and an anodecircuit as shown. An anode voltage supply battery 4 in series with aninductance element 5 is connected in the anode circuit of the tube 3.The grid circuit includes an inductance element 6 coupled to theinductance element 5 and a resistor 1 connected in series therewith. Inorder to effect frequency modulation of the oscillations generated withthe aid of the tube 3, the tube 2 is connected in parallel with theinductance element 6 and the resistor I so as to provide an effectiveimpedance in the oscillator circuit which is variable with variations inthe voltage impressed upon the control grid of tube 2.

The anode of oscillator tube 3 is capacitively coupled to the controlgrid of a vacuum tube ID by means of a condenser 8 and a resistor 9. Thetube I is operated as a limiting device for the purpose of providing anoutput signal of constant amplitude. A resistor connected in parallelwith a condenser I2 is provided in the cathode circuit of tube I0, inorder to produce the proper operating bias potential for this tube.

For the purpose of developing from the asymmetrical oscillations, whosefrequency is modulated in accordance with the modulation signals whichare supplied from the tube 3, sinusoidal oscillations similarlymodulated, there is provided in the anode circuit of the tube Ill aparallel resonant circuit l3 comprising an inductance element l and acondenser |4 tuned to the fundamental frequency of the oscillationssupplied by the oscillator tube 3.

In order to record the frequency-modulated carrier signal, there isprovided a suitable recording mechanism which may be of any conventionalconstruction. For example, as shown in Fig. 1, a physical medium such asa base in the shape of a wax disc may be disposed on a turntable 2|driven by a suitable motor 23, by Way of a shaft 22 and a suitablemechanical filter 24. The record 20 is thereby rotated about its axis ata substantially uniform speed. A cutting stylus 25 is supported in acutting head 26 which is associated with a screw 21 by means of whichthe stylus is moved across the disc 20 in substantially a radialdirection. The screw 21 is driven by the motor 23, being connectedtherewith by means of a gear train indicated generally at 28. Thecircular motion of the record 20 and the radial motion of the stylus 25cause spiral grooves to be cut into the surface of record 20.

In order to obtain a lateral recording of the impressions to berecorded, there is provided in the cutting head 26 a driving system ofany conventional type comprising, for example, a permanent magnet and acoil system, not shown, which is inductively coupled to the resonantcircuit |3 by means of an inductance element II.

In Fig. 2, there is shown schematically a system for reproducing audibleimpressions recorded upon a disc 33, which, for example, may be a recordmade by the apparatus of Fig. 1. The reproducing mechanism comprises aturntable 3| rotatably supporting the disc 33 and driven by a motor 32at a predetermined speed.

For the purpose of converting into electrical signals thefrequency-modulation characterics of the groove cut in the disc, thereis provided a stylus or needle 34, inserted in a conventional pickupdevice generally designated at 35 and preferably comprising a crystalarrangement housed within a ton arm 36. Such devices being well known inthe art, it is deemed unnecessary here to provide a detailed explanationthereof. The electrical signals produced by the pickup device areconducted by means of conductors 3T, 31 to the primary winding of amatching transformer 38. One side of the secondary winding of thetransformer 38 is grounded while the other side may be connected to aswitch 39 having two poles A and B as shown. Pole A is capacitivelycoupled to a parallel resonant circuit 4| comprising an inductanceelement 42 and a condenser 43, tuned to a frequency differing by apredetermined amount from the frequency-modulated carrier frequency.Preferably the resonant circuit is sharply tuned to a frequency higherthan the carrier frequency.

For the purpose of detecting the modulation signal, there is provided avacuum tube detector 44 having its control grid circuit connected acrossthe resonant circuit 4|. The cathode of the tube 44 is connected toground by way of a resistor 45 connected in parallel with a condenser46, across which arrangement the proper bias potential is developedforoperation of the tube as a detector. The anode circuit of tube 44includes an output resistor 4'! across which the detected signalappears, and a source of operating anode voltage as represented by thebattery 48.

For the purpose of amplifying the detected signal, there is provided anamplifier tube 5| capacitively coupled to the output resistor 41 by wayof a coupling condenser 49 and a grid leak resistor 53. Proper biasingpotential is obtained for the tube 5| by the provision of a suitablecathode resistor 52 by-passed by a condenser 53 as shown. The anodecircuit of tube 5| includes the primary winding of a transformer 54,across which the amplified signal is developed, connected in series witha suitable source of anode operating voltage or battery 55. Thesecondary winding of the transformer 54 is connected to the voice coil5'! of a loud speaker 58 comprising the voice coil and a membrane 59 bymeans of which the recorded audible impressions are reproduced.

In order to make the system just described also applicable to thereproduction of impressions recorded on discs in the conventionalmanner, there is provided a conductive connection between the pole B ofswitch 39 and the control grid of the amplifier tube 5|. With thisarrangement, by simply throwing the switch from the position shown inthe drawing to that indicated by broken lines, the tube 44 and itsassociated circuits are disconnected from the system and a substanetially conventional electrical phonograph is provided.

Referring now more particularly to the operation of the recording deviceshown in Fig. 1, audible signals are converted into electrical sig. nalsby means of the microphone I, which signals are applied to the controlgrid of the tube 2. The oscillator tube 3 in conjunction with inductanceelements 5 and 6 and resistor 1 disposed in its plate and grid' circuitsrespectively generates asymmetrical oscillations of a predeterminedfrequency. Since the discharge space of the tube 2 is connected inparallel with the series combination of the inductance elements 6 and 1and, since the operating characteristics of the tube' 2 change inaccordance with the signals applied to its control grid, the tube 2 andits associated elements function as an impedance in the oscillationcircuit whose value is varied in accordance with the voltage applied tothe grid of tube 2. The frequency of the oscillations developed by thegenerator 3, 5, 6, and I is thus varied in accordance with the signalsupplied to the tube 2.

The frequency-modulated asymmetrical oscillations are supplied, by wayof the coupling-condenser 8 and grid leak resistor 9, to the amplitudelimiting tube I0. The operating bias developed across the parallelcombination of the resistor II and the condenser I2 is of such valuethat the tube becomes saturated if the signals applied to its controlgrid exceed a certainpredetermined amplitude. The use of asymmetricaloscillations rather than sinusoidal oscillations is preferable since asinusoidal output signal of constant amplitude and predeterminedfundamental frequency can be more readily produced by amplitudelimitation of an asymmetrical input signal having the same fundamentalfrequency, than by amplitude limitation of a sinusoidal input signal ofthe same frequency. Hence input signals comprising impulses of varyingfrequency and having undesirable fluctuations in amplitude applied tothe limiting tube l0 produce output signals of constant amplitude andsinusoidal wave shape in the tuned circuit I3. This circuit is tunedsufficiently broadly to have a substantially uniform response over arange of frequency variations equal to the range over which theoscillations of the tube 3 are varied.

The signals developed across the tuned circuit |3 are applied to thecutting head 26 by means of the winding inductively coupled to theinductance element I5. The cutting head 26, being preferably of theconventional construction comprising a magnetic driving system, causesthe cutting stylus 25 to be displaced in accordance with the signalsapplied to the cutting head 26. The wax disc is rotated and the cuttinghead 26 is simultaneously moved in radial direction as previouslyexplained. In this manner, the cutter is guided along a predeterminedspiralshaped trace and cuts a groove oscillating with respect to thetrace with a constant amplitude and at a frequency which varies inaccordance with the frequency-modulated carrier signal applied to thecutting head 26.

There is thus produced a record representing impressions to be recordedcomprising a disc and a continuous spiral groove on its face, having anirregular inner surface varying in accordance with a carrier signalwhose frequency is modulated in accordance with the impressions to bereproduced.

Due to the fact that with this type of signal the groove irregularitiescut in the record are of uniform width and this width may be relativelysmall as compared with conventional records, it is possible to recordprograms of substantially greater length on a record of a given sizethan was heretofore possible.

In the operation of the reproducing device of Fig. 2, the disc 33,provided with spiral-shaped grooves having characteristics representinga frequency-modulated carrier signal, is rotated by means of theturntable 3| driven by the motor 32 at the same speed at which therecording was effected. The needle 34, supported by the tone arm 36,follows in lateral direction the path of the grooves in the well-knownmanner and its motion causes electrical signals to be produced by thepickup device 35. These signals are applied to the primary winding ofthe matching transformer 38 by means of conductors 3| guided inside thetone arm 36. The frequency-modulated carrier signal is then applied byway of pole A of switch 39 and coupling condenser 40 to the resonantcircuit 4|, which, as explained above, is sharply tuned to a frequencyhigher than the carrier frequency. Due to the fact that the impedancepresented by the resonant circuit 4| varies in accordance with theapplied signal frequency, the frequency-modulated carrier signal isconverted into an amplitude-modulated carrier signal which appearsacross the resonant circuit. Since the resonant circuit 4| is includedin the control grid circuit of the detector tube 44, detection of themodulation signal is effected by tube 44 and the modulation signalappears across the output resistor 41. This signal is applied, asexplained above, to the control grid of the amplifier tube 5| wherein itis amplified. The amplified signal appearing across the primary windingof the transformer 54 is induced in the secondary winding thereof and isapplied to the voice coil 51 of the loud speaker 58 where it actuatesthe membrane 59 to reproduce the audible impressions recorded upon thedisc 33.

If it is desired to reproduce recordings made in the conventionalmanner, the switch 39 is moved in the position to connect the secondaryWinding of the matching transformer 38 to the pole B. The electricalsignals derived from the pickup device 35 are then supplied directly tothe control electrode of the amplifier tube 5| since such recordedsignals do not require detection.

Preferably, the carrier frequency is chosen to be in the order ofbetween 15 and 20 kilocycles for a range of modulation frequencies ofbetween 20 and 5,000 cycles, and the frequencies to be transmitted maybe either symmetrically or asymmetrically disposed about the carrierfrequency in accordance with well-known principles of the art. All otherfrequencies can readily be prevented from actuating the loud speaker, byattenuating the same in the transmission path between the pickup deviceand the loud speaker. Attenuation of signals within the audible range isinherent in the use of the tuned circuit 4| as a means for convertingfrequency modulation into amplitude modulation, since the impedance ofthis tuned circuit is preferably negligible for frequencies lower thanthe lowest frequency which must be transmitted while this frequency isstill above the audible range. If desired, a bandpass filter ofconventional design may be inserted in the circuit between the pickupdevice 35 and the tuned circuit 4|. Since the background noise presentin conventional reproductions is due to erratic vibrations of the needle34 caused by roughness of the cut grooves and lies within the range ofaudible frequencies, and since such frequencies are not transmitted inthe reproducing system in accordance with the present invention, allbackground noise is eliminated.

In the foregoing, a system for recording and reproducing impressions oforal nature has been described. It is evident, however, that impressionsof visual and other nature can be likewise recorded and reproduced.Further, while the physical medium upon which characteristicsrepresenting the frequency-modulated carrier signal are recorded hasbeen described as a disc of the conventional type used for phonographicrecording, whereby the first recording is made on a wax disc serving asa master for the manufacture of other identical discs, it is obviousthat other physical media can be used, such as, for example,photographic films, magnetic tape, or the like, whereby characteristicssuch as transparency, magnetization and the like are varied inaccordance with a frequency-modulated carrier signal. It will beappreciated that all and any such arrangements are contemplated as apart of the present invention.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is therefore aimedin the appended claims to cover all such changes and modi- 'fications asfall within the true spirit and scope of the invention.

I claim:

1. A system for recording and reproducing impressions comprisingrecording apparatus including means for developing a carrier-frequencysignal, means for developing a modulation signal representingimpressions to be recorded, means for varying the frequency of saidcarrier-frequency signal in accordance with said modulation signal,means for recording on a physical medium characteristics representingsaid frequency-modulated carrier signal, and reproducing apparatusincluding means responsive to said characteristics for deriving saidfrequency-modulated carrier signal, means for deriving said modulationsignal from said carrier signal, and means for utilizing said derivedsignal to reproduce said impressions.

2'. An electric sound recording device comprising means for developing acarrier signal of superaudible frequency, means for developing anaudible modulation signal, means for varying the frequency of saidcarrier signal in accordance with said audible modulation signal andmeans for recording on a physical medium a continuous groove havingcharacteristics representing said superaudible frequency-modulatedcarrier signal.

3. An electric sound recording device comprising means for developing acarrier signal of superaudible frequency, means for developing anaudible modulation signal, means for varying the frequency of saidcarrier signal in accordance with said audible modulation signal andmeans for producing on a disc record said superaudiblefrequency-modulated carrier signal.

rier signal of sinusoidal Wave shape and substantially constantamplitude, and means for recording on a physical medium characteristicsrepresenting saidfrequency-modulated sinusoidal carrier signal.

5. An electric recording device comprising an electrical oscillator forgenerating a carrier-frequency signal of asymmetrical wave shape, meansfor developing a modulation signal representing impressions to berecorded, means for varying the frequency of said carrier signal inaccordance with said modulation signal, means for deriving from saidfrequency-modulated carrier signal of asymmetrical Wave shape afrequencymodulated carrier signal of sinusoidal wave shape and ofsubstantially constant amplitude, means for rotating a disc record at asubstantially uniform speed, a groove-cutting device, means for movingsaid groove-cutting device at a substantially uniform speed to cutspiral-shaped grooves into said disc record, and means for actuatingsaid groove-cutting device in accordance with said sinusoidalfrequency-modulated carrier signal.

6. A device for reproducing impressions from a record havingcharacteristics representing a carrier signal having its frequencymodulated in accordance with said impressions, comprising meansresponsive to said characteristics for deriving said frequency-modulatedcarrier signal, means for deriving said modulation signal from saidcarrier signal and means for utilizing said derived signal to reproducesaid impressions.

'7. A device for reproducing impressions from a record havingcharacteristics representing a carrier signal having its frequencymodulated in accordance with said impressions, comprising meansresponsive to said characteristics for deriving said frequency-modulatedcarrier signal, means for converting said frequency-modulated carriersignal into an amplitude-modulated carrier signal, means for derivingsaid modulation signal from said amplitude-modulated carrier signal andmeans for utilizing said derived modulation signal to reproduce saidimpressions.

8. A device for reproducing impressions from a disc record havingcharacteristics representing a carrier signal having its frequencymodulated in accordance with said impressions comprising meansresponsive to said characteristics for deriving said frequency-modulatedcarrier signal, means for converting said frequency-modulated carriersignal into an amplitude-modulated carrier signal, means for derivingsaid modulation signal from said amplitude-modulated carrier signal,means for amplifying said derived signal, and means for utilizing saidderived signal to reproduce said impressions.

9. A device for reproducing impressions from a disc record havingcharacteristics representing a carrier signal having its frequencymodulated in accordance with said impressions comprising means forrotating said disc record at a substantially uniform speed, pickup meansincluding a needle for deriving from said characteristics afrequency-modulated carrier signal characteristic of said impressions, atuned circuit for converting said frequency-modulated carrier signalinto an amplitude-modulated carrier signal, means for deriving themodulation signal from said amplitude-modulated carrier signal, andmeans for utilizing said modulation signal to reproduce saidimpressions.

10. A phonograph record representing sound impressions to be reproducedcomprising a base and a continuous spiral groove of wave form on saidbase varying in accordance with a carrier signal of superaudiblefrequency and having said frequency modulated in accordance with signalof audible frequency. I I

11. A phonograph record representing sound impressions to be reproducedcomprising a base and a continuous groove of wave form and of uniformamplitude on said base having variations on its inner surfacerepresenting a carrier signal of superaudible frequency and having saidfrequency modulated in accordance with a signal of audible frequency.

12. A device for reproducing impressions from records of a first typehaving characteristics representing an electrical carrier signal havingits frequency modulated in accordance with impressions and of a secondtype having characteristics representing an electrical signal ofmodulation frequencies representing said impressions, comprising meansresponsive to said record characteristics for deriving an electricalsignal represented by said characteristics, converting means fordeveloping from said derived frequency modulated carrier electricalsignals, a modulation signal representing the impressions to bereproduced, means for reproducing said impressions and means forselectively connecting said record characteristic responsive meanseither to said converting means or directly to said reproducing meansfor reproducing a record of said first type or said second typerespectively.

13. The method of recording and reproducing impressions which comprisesdeveloping a carrier-frequency signal, developing a modulation signalrepresenting impressions to be recorded, varying the frequency of saidcarrier-frequency signal in accordance with said modulation signal,recording characteristics representing said frequency-modulated carriersignal, utilizing said recorded characteristics to reproduce saidfrequency-modulated carrier signal, deriving said modulation signa1 fromsaid carrier signal, and utilizing said derived signal to reproduce saidimpressions.

PHILO T. FARNSWORTH.

